WITH A METHANOL INJECTION RECIPROCATING PUMP SYSTEM

ELEVATED PRESSURE PULSATION

Authors

Walter Kelm Engineer

Kelm Engineering, LLC Friendswood, TX, USA

Ray Kelm Chief Engineer

Kelm Engineering, LLC Friendswood, TX, USA

Methanol Injection Reciprocating Pump Pulsation Problem

• Offshore Floating Production Storage and Offloading (FPSO) high pressure methanol system

• Four quintuplex recip pumps in parallel • During commissioning, failure of Pressure Safety

Valve (PSV) components were observed • Likely issue was pressure pulsation

Discharge pressure of 323 barg

Relief valve (PSV) set pressure of 339 barg

Fixed Pump Speed: 446 RPM Flow Rate: 80 GPM per pump

Piping System Layout

Recip Pumps

Spillback pressure control valves

Continued piping to umbilical

PSV installed on the blind end of the pump

2 liter gas charged dampener

An acoustic study was done to review the pressure pulsation at the PSV body.

Acoustic Study Results Pressure pulsation due to pump A in operation

Pump A on the left.

Highest pressure pulsation in the pump manifold and at the PSV. Maximum of 73.2 bar peak-peak which is +/- 36.6 bar.

5-30 bar pk-pk pulsation in the piping, which is +/- 2.5-15 bar

Review of Acoustic Study Results

290

300

310

320

330

340

350

360

0 90 180 270 360

Pres

sure

, bar

g

Crank Rotation, degrees

Instantaneous Pressure at PSV

Pressure at PSV, barg PSV set pressure, barg API 674 Design Margin

API 674 3rd Ed C.1.7 specifies a 5% design margin between the maximum instantaneous pressure, including pulsation, and the PSV set pressure

Exceeds PSV set pressure by 16 bar

PSV set 5% above line pressure with 16 bar margin

Exceeds API 674 design margin by 31 bar

Acoustic Study Results

Baseline pulsation

Amplification due to acoustic response of piping

Pressure Pulsation at PSV Body

Pre

ssur

e P

ulsa

tion

Am

plitu

de

(bar

pea

k)

Pulsation over 16 bar peak opens PSV

Acoustic Study Results

Baseline pulsation

Pressure Pulsation at PSV Body

Pre

ssur

e P

ulsa

tion

Am

plitu

de

(bar

pea

k)

• Acoustic resonance results in amplified pulsation.

• Resonance occurs when a pump excitation frequency is near an acoustic natural frequency.

• There is broad amplification centered near 130 Hz due to manifold ¼ wave acoustic response.

• There are specific peaks at piping natural frequencies between 110-140 Hz.

Specific peaks at piping natural frequencies

Broad amplification near 130 Hz

Manifold ¼ Wave Acoustic Response

High pulsation at PSV (closed end)

Low pulsation at dampener (open end)

Acoustic length of pipe is 7.9 feet between areas with high and low pulsation

A pump manifold ¼ wave acoustic response is common for all pumps with a closed end on one side (PSV, blind flange, …) and an open end on the opposite side (gas or liquid dampener).

• ¼ wave acoustic natural frequency is a classic acoustic response with a closed end on one side and an open end on the other side.

• Wavelength, λ = 4 * La La is acoustic length λ = 31.6 ft La is 7.9 ft • Frequency, f = c / λ c is speed of sound f = 130 Hz c is 4108 ft/s • This equation works fairly well when there is a

clearly defined closed and open end. • But for complex piping, acoustic modeling

must be done to accurately calculate acoustic natural frequencies.

Manifold ¼ Wave Acoustic Response

Manifold ¼ Wave Acoustic Response

• For this application, the manifold ¼ wave frequency exists at 130 Hz due to the length of pipe from the PSV to the gas dampener.

• Due to the high pump speed (446 RPM), this frequency is near the 3rd and 4th pump harmonic(n=3,4)

fexcitation = RPM x 5 / 60 * n f3rd = 111.5 Hz f4th = 148.7 Hz

Possible Solutions • Improve margin between line pressure and

PSV set pressure – Increase PSV set pressure – Decrease line pressure

• Reduce pressure pulsation – Add an orifice near pump discharge – Add a gas charged dampener near PSV – Add liquid flow through style dampener to

reduce high frequency pulsation

Solutions: Improve PSV Margin • Increase PSV set pressure

– This was not possible because the PSV set pressure was selected due to the pipe pressure rating.

• Decrease line pressure – Lowering the line pressure by 36 bar would

satisfy the API 674 3rd Ed C.1.7 recommended margin

– However, the process requires a certain pressure

Solutions: Reduce Pressure Pulsation

• Add an orifice plate downstream of dampener – Pressure drop provides acoustic damping that

reduces amplification due to acoustic resonance • Add a gas charged dampener near the PSV

– Changes the acoustic response near the PSV • Add a liquid volume

– Liquid volume reduces high frequency pulsation.

Implemented Solution/Results Two changes were made to reduce the pressure pulsation and improve the margin

• A small gas charged dampener was installed on the PSV sense line to reduce the pressure pulsation measured by the PSV.

• The pump discharge set pressure was reduced to 302 barg.

These modifications have resulted in improved PSV life.

Lessons Learned • For a reciprocating pump, a PSV must be set

with a sufficient margin above line pressure. • API 674 recommends a 5% margin including

pressure pulsation. • Due to the manifold ¼ wave response,

pressure pulsation on the back side of a pump can be much higher than in the piping system.

• If a PSV is used on the back side of a pump, be sure to consider pulsation when choosing the PSV set pressure.